Alpha-methyldopa interference with the phosphotungstate uric acid test

The effect of alpha-methyldopa on the phosphotungstate method of uric acid analysis was tested using a group of 17 hypertensive patients being treated with only this drug for their elevated blood pressure. The uric acid values of these patients tested by the phosphotungstate method showed no significant difference from the uricase test values, when compared to a control population of 32 normotensive patients tested in the same manner. The proposed interference was further tested by in vitro studies. Both uricase and phosphotungstate analysis of uric acid was performed on serum containing various dilutions of alpha-methyldopa. The concentration of alpha-methyldopa required to clinically affect the uric acid level, whereby a false positive result occurred, was 60 mug/ml. The mean plasma aplha-methyldopa concentration in the study group, however, was found to be only 2.03 mug/ml. The postulated interference of therapeutic levels of alpha-methyldopa on the phosphotungstate uric acid method was invalid.